The present invention relates to an electrohydraulic three-port, two-position valve, especially for controls of mine roof supports operated by an oil-in-water emulsion, which comprises a valve rod reciprocatable in a longitudinal bore extending through the valve housing between two end positions by an electromagnet against a restoring force, in which the valve rod comprises on opposite sides of a central, reduced-diameter portion sealing members which cooperate with sealing members on the valve housing in such a manner that, in one end position of the valve rod, a consumer is connected to a tank and, in the other end position, to a pump, in which the consumer communicates with a portion of the bore in region of the reduced diameter of the valve rod, whereas the pump and the tank respectively communicates with radially increased bore portions respectively located in the region of the sealing members.
Such valves are for instance disclosed in German Offenlegungsschrift No. 29 23 719. This known valve is a magnet-operated, three-port, two-position valve with a spring return. The cooperating sealing members comprise in this known valve respectively two conical valve members mounted axially spaced from each other on the valve rod, as well as corresponding seat rings in the valve housing. The movement of the valve rod in one direction is carried out by means of an electromagnet with a long stroke, which acts parallel to the valve rod. The transmission of the force of the magnet armature to the valve rod is carried out by a two-arm tilting lever, mounted on a separate housing, in the relation of the lever arms of about 2:1. The valve rod is moved in the other direction by a coiled compression spring engaged in a hollow portion of the valve rod. The consumer communicates with the bore through the housing in the region of the central reduced-diameter portion of the valve rod, whereas the pump communicates with the bore in the region of the return trunnion, and the tank in the region of the trunnion connected to the magnet armature.
The above mentioned Offenlegungsschrift emphasizes that the valve disclosed therein should provide sufficiently large throughflow amounts. Such throughflow amounts require, however, for hydraulically operated mine roof supports nominal diameters of at least 10 mm. Experience has shown that at such nominal diameters the force for opening a seat valve of the aforementioned construction is about 4000 Newtons. Such a force would therefore be required in the present case, whereby not even the inherent friction forces and the return force of the coil compression spring are considered.
Since in underground mining operations the maximal electrical available power is about 20 watt, it cannot be assumed that in the known case with a magnet of long stroke the necessary moving force for the valve rod can be provided. In this case a tilting lever with a transmission ratio would be necessary, which would require dimensions for the housing which, for the control in an underground mine shaft, would be unrealistic. In addition, it has to be considered that this known valve requires a magnet with a long stroke. The force to be provided by a magnet depends, however, on its stroke. A magnet with a long stroke would therefore have such large force losses that, even with a tilting lever of a very high transmission ratio, the opening resistance of the valve could not be overcome.
Since in the above mentioned known valve the valve members have to be pressed with considerable force against the valve seats, in order to provide the necessary sealing pressure, there necessarily result, under further consideration of the friction, relatively high displacement forces. On the other hand, for safety reasons in underground operations there are only very small operating forces for the control magnets available. Therefore, in order to operate the known three-port, two-position valve, a precontrol valve would be necessarily required. In this case, the magnet would operate a valve with small nominal diameter, and the small volume stream produced thereby would have to be directed to a control piston which would operate the valve rod. Such a valve combination is, however, not only very expensive, but also liable to malfunction. Such precontrol valves can hardly be used at the rough operating conditions prevailing in underground mine operations.
In order to avoid the high operating forces inherent with seat valves, sleeve valves, as for instance disclosed in German Offenlegungsschrift No. 28 08 447, have already been used for the control of mine roof supports operated by an oil-in-water emulsion. The construction of such a sleeve valve requires, however, a large number of seals between the sleeve and the valve housing. Thereby high friction resistance will necessarily be developed which, as in the above mentioned seat valve disclosed in German Offenlegungsschrift No. 29 23 719, will hardly permit a direct movement of the sleeve or slide by a magnet, especially when this valve has to be constructed for a nominal diameter of least 10 mm. If further the return force is considered, which acts against the force applied by the operating magnet, it will be evident that in this case the operating magnet would have to be dimensioned so large that it would be uninteresting for operating an underground mine roof support. Therefore, in this case a direct magnet control is impossible, and only a complicated, expensive precontrol, liable to malfunction, is usable.